Group merge (#1039)

This PR adds a mechanism to hide groups but have their options visible
as part of the parent.
This works for option group names starting with a '+'

for example 
```
 CLI::App app;

    bool flag = false;
    std::optional<bool> optional_flag = std::nullopt;

    app.add_option("--tester");
    auto *m1=app.add_option_group("+tester");

    m1->add_option("--flag", flag, "description");
    m1->add_option("--optional_flag", optional_flag, "description");

    CLI11_PARSE(app,argc, argv);
```
will produce help as 
```txt
Options:
  -h,--help                   Print this help message and exit
  --tester
  --flag BOOLEAN              description
  --optional_flag BOOLEAN     description
```

instead of 
```
Options:
  -h,--help                   Print this help message and exit
  --tester
[Option Group: tester]
  Options:
    --flag BOOLEAN              description
    --optional_flag BOOLEAN     description

```

Fixes issue #1034 and a few other past issues or questions

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## Unreleased
+
+- add mechanism to allow option groups to be hidden and all options be
+  considered part of the parent for help display
+
 ## Version 2.4: Unicode and TOML support
 
 This version adds Unicode support, support for TOML standard including multiline

README.md

@@ -1205,7 +1205,17 @@ auto hidden_group=app.add_option_group("");
 ```
 
 will create a group such that no options in that group are displayed in the help
-string.
+string. For the purposes of help display, if the option group name starts with a
+'+' it is treated as if it were not in a group for help and get_options. For
+example:
+
+```cpp
+auto added_group=app.add_option_group("+sub");
+```
+
+In this case the help output will not reference the option group and options
+inside of it will be treated for most purposes as if they were part of the
+parent.
 
 ### Configuration file
 

include/CLI/App.hpp

@@ -1340,6 +1340,11 @@ class Option_group : public App {
         : App(std::move(group_description), "", parent) {
         group(group_name);
         // option groups should have automatic fallthrough
+        if(group_name.empty() || group_name.front() == '+') {
+            // help will not be used by default in these contexts
+            set_help_flag("");
+            set_help_all_flag("");
+        }
     }
     using App::add_option;
     /// Add an existing option to the Option_group

include/CLI/impl/App_inl.hpp

@@ -784,7 +784,14 @@ CLI11_INLINE std::vector<const Option *> App::get_options(const std::function<bo
                                      [&filter](const Option *opt) { return !filter(opt); }),
                       std::end(options));
     }
-
+    for(const auto &subcp : subcommands_) {
+        // also check down into nameless subcommands
+        const App *subc = subcp.get();
+        if(subc->get_name().empty() && !subc->get_group().empty() && subc->get_group().front() == '+') {
+            std::vector<const Option *> subcopts = subc->get_options(filter);
+            options.insert(options.end(), subcopts.begin(), subcopts.end());
+        }
+    }
     return options;
 }
 
@@ -798,7 +805,13 @@ CLI11_INLINE std::vector<Option *> App::get_options(const std::function<bool(Opt
             std::remove_if(std::begin(options), std::end(options), [&filter](Option *opt) { return !filter(opt); }),
             std::end(options));
     }
-
+    for(auto &subc : subcommands_) {
+        // also check down into nameless subcommands
+        if(subc->get_name().empty() && !subc->get_group().empty() && subc->get_group().front() == '+') {
+            auto subcopts = subc->get_options(filter);
+            options.insert(options.end(), subcopts.begin(), subcopts.end());
+        }
+    }
     return options;
 }
 

include/CLI/impl/Formatter_inl.hpp

@@ -180,10 +180,10 @@ CLI11_INLINE std::string Formatter::make_subcommands(const App *app, AppFormatMo
     std::vector<std::string> subcmd_groups_seen;
     for(const App *com : subcommands) {
         if(com->get_name().empty()) {
-            if(!com->get_group().empty()) {
+            if(com->get_group().empty() || com->get_group().front() == '+') {
-                out << make_expanded(com);
+                continue;
             }
-            continue;
+            out << make_expanded(com);
         }
         std::string group_key = com->get_group();
         if(!group_key.empty() &&

tests/OptionGroupTest.cpp

@@ -221,6 +221,32 @@ TEST_CASE_METHOD(TApp, "BasicOptionGroupMin", "[optiongroup]") {
     CHECK(std::string::npos != exactloc);
 }
 
+TEST_CASE_METHOD(TApp, "integratedOptionGroup", "[optiongroup]") {
+    auto *ogroup = app.add_option_group("+clusters");
+    int res{0};
+    ogroup->add_option("--test1", res);
+    ogroup->add_option("--test2", res);
+    ogroup->add_option("--test3", res);
+    int val2{0};
+    app.add_option("--option", val2);
+    ogroup->require_option();
+
+    args = {"--option", "9"};
+    CHECK_THROWS_AS(run(), CLI::RequiredError);
+
+    args = {"--test1", "5", "--test2", "4", "--test3=5"};
+    CHECK_NOTHROW(run());
+
+    auto options = app.get_options();
+    CHECK(options.size() == 5);
+    const CLI::App *capp = &app;
+    auto coptions = capp->get_options();
+    CHECK(coptions.size() == 5);
+    std::string help = app.help();
+    auto exactloc = help.find("clusters");
+    CHECK(std::string::npos == exactloc);
+}
+
 TEST_CASE_METHOD(TApp, "BasicOptionGroupExact2", "[optiongroup]") {
     auto *ogroup = app.add_option_group("clusters");
     int res{0};